This invention relates to a semiconductor device with a semiconductor element (such as a photo semiconductor used for detecting a substance, for optical communication, for optical fiber communication, or as a photo coupler) sealed with a resin, and a method for manufacturing the semiconductor device.
There is a photo semiconductor device, in which a photo semiconductor element (such as a light emitting semiconductor element or a light receiving semiconductor element) is mounted on a carrier member such as a lead frame or a TAB (tape automated bonding) tape, the carrier member with the photo semiconductor element is contained in the cavity of a die, and the cavity with the carrier member and the semiconductor element is filled with a resin, thereby forming a sealed package. In general, an epoxy resin, which is a thermosetting resin, is used as a filling resin for avoiding cracks.
FIGS. 8, 9A and 9B show a photo semiconductor device 10 as an example of a semiconductor device of this type. The photo semiconductor device 10 comprises a first lead frame 11 with a thickness of about 0.4 mm, a second lead frame 12 with a thickness of about 0.4 mm, a semiconductor element 13 with a thickness of 300-400 .mu.m, square in shape(0.28 mm.times.0.28 mm), attached to the first lead frame 11, and a package member 14 sealing the first and second lead frames 11 and 12 and the semiconductor element 14. Reference numerals 15a and 15b denote silver films with a thickness of about 10 .mu.m, and reference numerals 16a and 16n solder films with a thickness of about 20 .mu.m. Each solder film 16a is separated by about 1 mm from the package member 14.
The photo semiconductor device 10 is manufactured by the following process. Distal end portions 11a and 12a of the first and second lead frames 11 and 12 are beforehand plated with silver to thereby form the silver films 15a and 15b.
Then, the first lead frame 11 with the semiconductor element 13 mounted thereon, and the second lead frame 12 are put in a cavity 22 defined by upper and lower mold components 20 and 21. The semiconductor element 13 is connected to the second lead frame 12 by an aurum bonding wire 13a. Subsequently, an epoxy resin T as a thermosetting resin is injected into the cavity through a gate 22. Thereafter, the solder films 16a and 16b are formed in a dip process in which proximal end portions 11b and 12b of the first and second lead frames 11 and 12 are dipped in a solder bath.
The epoxy resin is widely used since it shows a high fluidity when it is molded, and a high adhesion to the first and second lead frames 11 and 12 after it is hardened. However, about 180 seconds are required to harden the thermosetting resin such as the epoxy resin requires after molding, which will result in a low productivity and may cause a burr.
To shorten the time required for hardening and increase the productivity, it is considered to use a thermoplastic resin as the sealing resin. The thermoplastic resin is useful in increasing the productivity, since it will be hardened in only about 10 seconds.
However, the following problems will occur when the cavity 22 is filled with the thermoplastic resin to seal the semiconductor element 13.
First, the thermoplastic resin shows a lower adhesion to the lead frames than the epoxy resin. Therefore, moisture cannot completely be prevented from entering the device through the boundaries of the first and second lead frames and the package member 14. Thus, the resulting device has a low moisture tolerance.
Second, in the dip process using solder, it is possible that the package member 14 will be contaminated with solder, that a lower portion of the package 14 will be softened by the heat of solder and then deformed due to its own weight, or that moisture contained in the resin will be converted into bubbles as a result of evaporation.
Last, when the first and second lead frames 11 and 12 are coated with a flux by dipping, etc. to facilitate the forming of the solder films, the flux will enter the interior of the semiconductor device through the boundaries of the first and second lead frames 11 and 12 and the package member 14, thereby contaminating the semiconductor element 13 and reducing the reliability of the device.
On the other hand, when the cavity is filled with the sealing resin, the resin will flow around the bonding wire 13a in directions indicated by arrows .alpha. and .beta. in FIGS. 9A and 9B. This being so, the bonding wire 13a will greatly be deformed due to the flow resistance of the sealing resin, if the thermoplastic resin, which has a high viscosity, is used as the sealing resin. In particular, the flow in the direction .alpha. will greatly deform the bonding wire 13a, with the result that the wire 13a may well be cut or be brought into contact with the first lead frame 11. Thus, it is difficult to impart a sufficient reliability to the photo semiconductor device.
Japanese Patent Application KOKAI Publication No. 6-283645 discloses a case where the overall surface of the lead frame is coated with an epoxy resin-based conductive paste. In this case, however, it is possible that the moisture permeating the paste outside the package (the epoxy resin has a high hygroscopicity) will corrode the bonding portion of the lead frame which is located within the package. If, in this case, a solder film is formed on the overall surface of the lead frame in place of the conductive paste, bonding cannot be performed.